The present invention relates to heat exchangers and more particularly to the formation of the folded fins in a heat exchanger.
Heat exchangers for a wide variety of applications, such as radiators, heater cores, condensers, and evaporators are well known in the art and are generally of a similar basic construction. The basic construction of such a heat exchanger typically includes two headers for the input and output of a heat exchanging liquid. A plurality of regularly spaced tubes extend between the two headers to permit the heat exchanging liquid to flow therebetween. A plurality of heat conductive fins, also known as an air center, occupy spaces between each pair of adjacent tubes and are oriented to permit a flow of air therethrough. Most typically, the fins are constructed as a convoluted folded fin, which when viewed from one edge are folded in an accordion-like pattern. The folded fins are bonded, most typically metallurgically bonded, to the sides of the tubes to enhance the heat transfer from the liquid flowing in the tubes to the folded fins so that the excess heat can be convectively transferred to an air stream flowing through the fins. Features of a typical prior art heat exchanger core 10 are illustrated in FIGS. 1 and 2.
Prior art heat exchanger core 10 typically comprises a plurality of tubes 12 having an elliptical shape wherein the width of tube ends 16 are generally smaller than the width of a central section 14 of tube 12 such that central section 14 forms a compressible crown. A plurality of air centers 18 formed as accordion-like folded fins are placed between adjacent ones of tubes 12. Each air center 18 is generally comprised of a plurality of adjacent convolution legs 24, wherein each convolution leg 24 is connected to a previous leg 24 by top tip radius 20 and to a subsequent convolution leg 24 by bottom tip radius 22. Each convolution leg 24 can further have a plurality of louvers 26 formed therein for improved heat transfer properties. Prior art core 10 is assembled by abutting a plurality of tubes 12 and air centers 18 in an alternating fashion such that the tip radii 20, 22 of an air center 18 are abutted to facing sides of adjacent tubes 12. Since tubes 12 have center sections 14 that are generally wider than tube ends 16, the arranged tubes 12 and air centers 18 can then be compressed to form core 10 to a desired dimension. By compressing the stacked tubes 12 and air centers 18, tip radii 20 and 22 can be caused to substantially contact the facing sides of tubes 12. The compressed core 10 is then processed to bond tip radii 20 and 22 to tubes 12, most typically by a metallurgical bonding process.
Each air center 18 in prior art core 10 is generally formed by passing a strip of heat conductive metal through a pair of intermeshing star-shaped form rolls similar to the rolls illustrated in FIG. 6. The intermeshing of the star-shaped form rolls form the generally flat metallic strip into an accordion-like folded fin. A partial cross-section of a prior art form roll 30 is illustrated in FIG. 3 wherein FIG. 3 corresponds to the cross-section along the lines 7xe2x80x947 of FIG. 6. Form roll 30 is generally comprised of a plurality of discs 32-36 wherein each disc forms a specific portion of convolution legs 24 and tip radii 20 and 22. Star-shaped discs 32-36 have a plurality of teeth about the circumference wherein each tooth has a top edge 38 and a valley 39 is defined at the bottom convergence of adjacent teeth. The teeth of outer discs 32 and 36 have a plain face 40 to form ends 25 of leg 24. Discs 33 and 35 are positioned interiorly of discs 32 and 36, and their teeth have faces 42 for forming louvers 26 in leg 24. Stripper disc 34 is positioned between discs 33 and 35. Stripper disc 34 has no teeth and has a diameter substantially less than the diameter circumscribed by valleys 39. Stripper disc 34 in combination with adjacent discs 33 and 35 define a gap 44 between discs 33 and 35 to permit a stripper finger 46 to be at least partially contained therein. Stripper finger 46 facilitates the removal of formed air center 18 from form roll 30, and thus remains below valleys 39 to permit the proper intermeshing of teeth from the discs of the two intermeshing form rolls.
The intermeshing form rolls produce three different specific characteristics of the air center 18; the angle of louvers 26, the height of air center 18, and the size of tip radii 20 and 22. The form rolls operate under minimal clearance to produce the desired effect onto the blank heat conductive strip. The placement of stripper disc 34 corresponds to middle turnaround 28 of air center 18. Stripper disc 34 does not come in contact with air center 18 but allows for the clearance of stripper finger 46 to enter form roll 30 without creating an interference with the heat conductive strip being formed thereon. The heat conductive strip is trapped by the mating top and bottom form rolls 30 and is in turn drawn over the corresponding edges 38 of the discs to form the top and bottom tip radii 20 and 22. As the clearance between form rolls is reduced, the bend radii 20 and 22 of the heat conductive strip are also reduced, thereby resulting in sharper tip radii 20 and 22. As tip radii 20 and 22 become sharper, the height of air center 18 correspondingly becomes higher. The gap 44 created by stripper disc 34 allows that portion of the heat conductive strip in the region between discs 33 and 35 to be pushed toward gap 44 rather than forming a clean bend at the radius. This interaction results in middle turnaround 28 to be at a higher height than the rest of each individual top and bottom tip radii 20 and 22. Thus, as the mating top and bottom form rolls are setting the corresponding tip radii between them, the material at gap 44 is formed at a sharper radius, resulting in a higher center height of middle margin turnaround 28 of air center 18.
During assembly, core 10 is compressed to meet a predetermined core package dimension prior to placing a header on the ends of tubes 12. The height of air centers 18 should be substantially constant from convolution to convolution since a center of excessive height will cause air center 18 to collapse. Similarly, a center height that is too low will cause the air centers 18 to drop out of the core block 10 prior to bonding air centers 18 to tubes 12. The higher middle margin turnaround 28 could thus interfere with the proper assembly of core 10. However, as previously discussed, tubes 12 have a compressible crown 14 to permit some compression of tube 12 during assembly. This compressibility allows the increased height of air center middle margin turnaround 28 to be absorbed by the compression of tubes 12.
In the past, tubes 12 have been fabricated of either welded or extruded construction. However, a folded tube 50 as shown in FIG. 4 has now been introduced into the construction of heat exchanger cores. Folded tube 50 is designed and formed to have legs 52 and 54 in middle section 56 of tube 50. Legs 52 and 54 are non-compressible thereby removing the flexibility exhibited by tube 12 having a compressible crown center section 14. The non-compressibility of center section 56 results from legs 52 and 54 bottoming out on an opposite side of the folded tube 50. The tube sections between middle portion 56 and ends 58 do however, retain a degree of compressibility. Since legs 52 and 54 align directly with the high middle margin turnaround 28 of air center 18, the excess height of middle margin turnaround 28 cannot be compensated for since center portion 56 is no longer compressible. Therefore, assembling a heat exchanger core from tubes 50 in combination with air centers 18 having an increased height middle margin turnaround 28 provide additional difficulties in insuring contact between the tip radii 20 and 22 with the sides of tubes 50 during assembly of the core 10. Further, the combination of the middle margin turnaround 28 height with the non-compressible middle portion 56 height of tube 50 can cause collapsed air centers or a poor bond therebetween if the center height is too low.
Thus, there is a need for a method of forming an air center wherein its middle margin turnaround is at or below the height of the remainder of the tip radii.
One aspect of the present invention is an improved air center form roll for use in combination with a like form roll to produce an accordion-like folded fin for a heat exchanger core. The form roll being of the type comprising a plurality of form discs abutted one to the other and having a plurality of star-like teeth thereabout. Points of the star-like teeth define a major diameter of the form roll and valleys between adjacent ones of the star-like teeth define a minor diameter of the form roll. The form roll further includes at least one stripper disc having no teeth therearound and interposed between at least two of the plurality of form discs wherein the stripper disc has a diameter less than the minor diameter of the form roll. The improvement to the form roll comprises a modified disc having a plurality of modified star-like teeth thereabout. The modified disc is interposed at each abutment between the stripper disc and one of the abutting form discs. The modified disc further having a major diameter defined by points of the modified star-like teeth, the major diameter of the modified disc being smaller than the major diameter of the plurality of form discs.
Another aspect of the present invention is a method of improving a form roll utilized to form a folded fin for use in a heat exchanger core. The form roll being of the type comprising a plurality of form discs abutted one to the other and having a plurality of star-like teeth thereabout. Points of the star-like teeth define a major diameter of the form roll and valleys between adjacent ones of the star-like teeth define a minor diameter of the form roll. The form roll further includes at least one stripper disc having no teeth therearound and interposed between at least two of the plurality of form discs to abut with the two form discs. The stripper disc has a diameter less than the minor diameter of the form roll. The method includes the steps of fabricating a plurality of modified discs, each modified disc having a plurality of modified star-like teeth thereabout and further having a major diameter defined by points of the star-like teeth, the major diameter of the modified disc being smaller than the major diameter of the plurality of form discs. One modified disc is interposed between each abutted form disc and stripper disc, and the star-like teeth of the modified disc are aligned with the star-like teeth of the form discs.
Yet another aspect of the present invention is a method of making a folded fin air center having a reduced height middle margin turnaround for use in a heat exchanger core. The method includes the steps of providing a pair of form rolls, each form roll being of the type comprising a plurality of form discs abutted one to the other and having a plurality of star-like teeth thereabout. Points of the star-like teeth define a major diameter of the form roll and valleys between adjacent ones of the star-like teeth define a minor diameter of the form roll. The form roll further includes at least one stripper disc having no teeth therearound and interposed between at least two of the plurality of form discs. The stripper disc has a diameter less than the minor diameter of the form roll. The form discs abutting the stripper disc are modified form discs having modified star-like teeth therearound. Points of the modified teeth define a major diameter of the modified disc to be less than the major diameter of the form roll. The form rolls are rotated in opposite directions in a manner to cause the star-like teeth of one form roll to intermesh with the star-like teeth of the other form roll. A blank strip of heat conductive material is fed between the rotating form rolls. The teeth of each form roll are allowed to engage the blank strip. The blank strip is then formed over the point of an opposing tooth to form a tip radius adjoining two legs of a folded fin. The strip is caused to be further drawn over the point of the opposing tooth to form a leg having a middle margin turnaround in the region over the modified discs and the stripper disc wherein the height in this region is less than the height of the remainder of the leg over the region of the plurality of form discs.